The invention relates to an expandable shell, especially one made of sheet metal, with mutually overlapping longitudinal edges for repairing the insides of sewer pipes, the shell comprising two end portions and one intermediate portion combining said end portions into a single continuous element, with, allocated to each one of said end portions, an interlocking closure permitting the diameter of the shell to be enlarged but forming a positive locking engagement in the opposite direction, said interlocking closure having a slot and ridge arrangement beginning adjacent to one longitudinal edge and extending in a circumferential direction with slots and/or ridges parallel to the axis.
A shell of this kind is known from the PCT application WO 95/15460. Similar prior art can be found in the German utility models DE-U-8707049 and 9313379 and in the European patent application EP-A-0101258. Shells of this kind have a sealing means and/or an adhesive applied to their outer sides after a packer has been inserted into them. A traction cable is used to haul the packer, together with the shell surrounding it, into position in the sewer pipe, where the packer is inflated by means of an entrained pneumatic hose, so that the shell expands in the process and remains pressed against the section of pipe to be repaired. The interlocking closure ensures that the shell remains in position even after the packer has been withdrawn.
Shells of the known type can be used successfully in straight sewer pipes in order, for example, to seal cracks in the wall. In practice, however, damage to sewer pipes often occurs where two neighbouring sections of pipe are radially offset. In many cases, the offset is bridged by a connecting sleeve. Damage of this kind cannot be repaired with the known repair shells, because, when in an interlocked position supported from the outside, the shells exhibit a high resistance to distortion similar to that of a circumferentially closed pipe, so that they cannot be bent. The shells would remain in an angular position in the two sections of sewer pipe to be joined; they would be unable to expand to the internal diameter of the sections of sewer pipe and would only be in contact with the sewer pipes at certain points.